all: use T.TempDir to create temporary test directory (#8280)

This commit replaces `os.MkdirTemp` with `t.TempDir` in tests. The
directory created by `t.TempDir` is automatically removed when the test
and all its subtests complete.

Prior to this commit, temporary directory created using `os.MkdirTemp`
needs to be removed manually by calling `os.RemoveAll`, which is omitted
in some tests. The error handling boilerplate e.g.
	defer func() {
		if err := os.RemoveAll(dir); err != nil {
			t.Fatal(err)
		}
	}
is also tedious, but `t.TempDir` handles this for us nicely.

Reference: https://pkg.go.dev/testing#T.TempDir
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

lib/config/config_test.go

@@ -504,13 +504,10 @@ func TestFolderPath(t *testing.T) {
 }
 
 func TestFolderCheckPath(t *testing.T) {
-	n, err := os.MkdirTemp("", "")
-	if err != nil {
-		t.Fatal(err)
-	}
+	n := t.TempDir()
 	testFs := fs.NewFilesystem(fs.FilesystemTypeBasic, n)
 
-	err = os.MkdirAll(filepath.Join(n, "dir", ".stfolder"), os.FileMode(0777))
+	err := os.MkdirAll(filepath.Join(n, "dir", ".stfolder"), os.FileMode(0777))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -602,11 +599,7 @@ func TestWindowsLineEndings(t *testing.T) {
 		t.Skip("Windows specific")
 	}
 
-	dir, err := os.MkdirTemp("", "syncthing-test")
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll(dir)
+	dir := t.TempDir()
 
 	path := filepath.Join(dir, "config.xml")
 	os.Remove(path)